1. Field of the Invention
The present invention relates to a screw head structure.
2. Description of the Related Art
In a conventional injection molding device, resin heated and fluidized in the heating cylinder is injected under high pressure into the mold, the resin in the mold is cooled to be solidified or cured, then the mold is opened in order to take out the molded article. Such a conventional injection molding device has a screw provided in the heating cylinder so as to be rotatable and movable forward and backward. The screw is moved forward to inject melted resin.
The screw has on its front end a screw head for pushing out resin. The screw head is connected to the screw by internal screw engagement.
FIG. 2 is a sectional view of a conventional screw head structure of an injection molding device. FIG. 2 shows: a screw head 1; a counter-flow stopper ring 2 for stopping resin from flowing back; a seal ring 3 for abutting on the counter-flow stopper ring 2 when the counter-flow stopper ring 2 is moved back; a screw 4; and a heating cylinder 5.
After the cavity of the mold is filled with resin by moving the screw 4 forward, a metering step is then performed in which resin for the next injection is melted and plasticized and is stored in space 6 around the tip of the screw head 1. In the metering step, resin supplied down from a hopper through a material inlet is conveyed forward by rotation of the screw 4 through a groove 4a formed on the screw 4. The resin is melted in the heating cylinder 5 while being conveyed therein, and then stored in the space 6 around the tip of the screw head 1. Melting of the resin generates pressure inside the heating cylinder. The pressure acts on the screw 4 as a reaction force such that the screw 4 is moved backward.
The resin thus stored in the space 6 around the tip of the screw head 1 is injected into the mold through an injection nozzle (not shown) by the screw 4 driven forward by the driving means.
Due to the pressure caused when the resin is injected into the mold, a portion of the resin would flow back through the groove 4a of the screw 4 if this were not prevented by the counter-flow stopper ring 2. The counter-flow stopper ring 2 is provided on a front end portion of the screw 4. To prevent the counter-flow stopper ring 2 from coming apart from the screw 4, the screw head 1 has an outside diameter greater than the inside diameter of the stopper ring 2. The screw 4 and the screw head 1 are coupled by engagement between a female screw 4b formed in the screw 4 and a male screw 1a formed on a rear end portion of the screw head 1.
The counter-flow stopper ring 2 is freely rotatable with respect to the screw head 1 (an individually rotatable counter-flow stopper ring).
In the screw head structure constructed as above, during the metering step, the counter-screw stopper ring 2 is moved forward to abut on the screw head 1 due to pressure caused by the resin conveyed forward as the screw 4 is rotated and moved back. The resin between the inner periphery of the counter-flow stopper ring 2 and the outer periphery of the screw head 1 is conveyed through abutting portions of the counter-flow stopper ring 2 and the screw head 1. Because the screw head 1 is rotated while the counter-flow stopper ring 2 is not rotated, the resin is overheated by friction heat caused by the relative rotation therebetween, resulting in burned resin. Further, friction between the abutting portions of the screw head 1 and the abutting portions of the counter-flow stopper ring 2 causes the abutting portions to abrade.
To eliminate such problems, another screw head structure has been developed in which a counter-flow screw is rotatable together with a screw head (a co-rotatable counter-flow stopper ring).
FIG. 3 is a sectional view of such a conventional screw head structure of an injection molding device. The figure shows: a screw head 1, a seal ring 3, a screw 4, a heating cylinder 5, space around the tip of the screw head 1, and a counter-flow stopper ring 10. Two stopper grooves 11 for restricting rotation of the counter-flow stopper ring 10 are formed on the periphery of the screw head 1. The two stopper grooves 11 are circumferentially arranged. Thus, when the screw head 1 is rotated together with the screw 4, the stopper grooves 11 abut against the counter-flow stopper ring 10 so that the screw head 1 and the counter-flow stopper ring 10 rotate together.
In the conventional screw head structure constructed as above, resin burning will not occur because the counter-flow stopper ring 10 rotates together with the screw head 1. However, twisting torque occurs between the screw head 1 and the stopper ring 10 because while the screw 4 is rotated by the driving means, the screw head 1 receives a reaction force from the counter-flow stopper 10 which hinders the screw head 1 from rotating. Therefore, a portion which fixes the screw head 1 to the screw 4 may sometimes break.